In elevator hoisting machines, for example, a machinery brake that mechanically connects with a rotating part of the hoisting machine is normally used as a brake device. Structurally, the machinery brake can be e.g. a drum brake or a disc brake.
The machinery brake is activated by disconnecting the current supply of the electromagnet of the machinery brake. When disconnecting the current supply of the electromagnet, the springs in the brake push the armature part, which is provided with a brake pad, against the braking surface of the rotating part to brake the movement of the rotating part of the hoisting machine.
The machinery brake is opened by supplying current to the electromagnet of the machinery brake. The brake opens when the electromagnet pulls the armature part off the braking surface of the rotating part of the hoisting machine by resisting the thrusting force produced by the springs. During a run, current is connected to the electromagnet, in which case the brake is open and the elevator car can move up or down in the elevator hoistway. The brake implementation of an elevator can be e.g. such that the same hoisting machine comprises two or more machinery brakes.
As the current of the electromagnet decreases, the force exerted by the spring finally exceeds the force of attraction of the electromagnet, and the brake activates. Owing to the imbalance of forces the brake pad strikes against the braking surface of the rotating part of the machine. When the brake opens, the electromagnet again exerts on the armature part a force that is in the opposite direction to the spring force. When the force exerted on the armature part by the electromagnet grows to be greater than the spring force, the air gap between the frame part and the armature part closes, and the armature part strikes against the frame part.
Collision of the metallic parts of a brake against each other when the brake is activated or opened may cause a disturbing noise. The noise problem is eliminated whenever possible by adding e.g. a separate damper to the air gap between the frame part and the armature part, which damper when the brake is open prevents direct contact between the metal surfaces of the frame part and of the armature part. A damper can be made of an elastic material, such as rubber or polyurethane; a damper can also be implemented with a separate spring dimensioned for this purpose, such as with a helical spring or cup spring.
The air gap between the frame part and the armature part can vary e.g. owing to manufacturing tolerances of the damper. The force of attraction of the electromagnet decreases as the air gap increases, in which case opening/keeping open the brake requires more current than before. At the same time the opening delay of the brake increases. Incorrect dimensioning of the magnitude of the air gap/the damping force of a damper can, on the other hand, also result in a deterioration of the noise damping property of the brake. Owing to the aforementioned reasons, among others, the operating range of a damper of a brake must be set quite precisely.
Adjustment of the operating range of a damper usually takes place in connection with the manufacturing of the brake. In one embodiment cavities are machined in the countersurface of the armature part on the air gap side, into which cavities elastic dampers are fitted. After this the height of the dampers with respect to the plane of the countersurface is machined to a predefined value. One problem of the solution is, on the other hand, the variation of the material properties of the damper, owing to which also the damping force brought about by the damper varies; also variation caused by the cutting tolerances of the damper as well as by the machining precision of the grinding causes divergence in the damping force/magnitude of the air gap.
In a second embodiment the magnitude of the air gap between the countersurfaces of the frame part of the brake and of the armature part is measured with a dial indicator, and the distance of the dampers from the armature part is adjusted with a tightening bolt in the frame part. Adjustment of the damping is performed as manual work using a dial indicator in the corners of the countersurfaces and at the same time adjusting on an auditory basis the volume of the noise produced by opening of the brake. Adjustment of the damping of the brake is time-consuming and also the quality of the manual work might vary.
Inaccuracy of the adjustment of the damping force results in the damping forces achieved by the different dampers differing from each other. In this case some dampers are loaded more than others, owing to which also the countersurface(s) of the brake is/are loaded unevenly. Owing to the uneven loading, the air gap between the frame part and the armature part is not always equal at the different points of the countersurfaces. Variation of the air gap might, among other things, weaken the force of attraction produced by the electromagnet and, on the other hand, might also cause contact between the countersurfaces of the frame part and of the armature part, in which case the noise damping property of the brake deteriorates.